Magnetrons for known domestic ovens are provided with an L-C filter to prevent, as far as is possible, stray radiation generated by the magnetron from passing along the leads which supply power to the cathode heater. Such a filter, which is located at least partially within a screen chamber housing the magnetron terminals, is known from U.S. Pat. No. 4,900,985.
A typical domestic cooker magnetron has a peak power of a few kilowatts, and an average power of around 1 kW and requires a heater current of around 10 A. However, for industrial RF processing applications, peak powers of several tens of kilowatts are needed, and a correspondingly larger heater supply is needed with typical currents of the order of 100 amps, so that much higher gauge conductors are needed compared with domestic cooker magnetrons. In particular, it would not be practical or economic to wind such high gauge conductors into a choke coil used for a domestic cooker magnetron.
A basic problem to be addressed is therefore that in a microwave source for industrial applications a magnetron requires a high voltage supply to be applied to the cathode, perhaps as much as −20 kV, together with a heater supply of typically 11 V at 110 A, derived from an isolation transformer (and rectifier if a DC heater is used) connected across heater and cathode terminals of the magnetron. These terminals can be the source of considerable stray radiation in the frequency range 100 MHz to >1 GHz, as illustrated in a first inset 20 in FIG. 1, for a magnetron designed to produce an output at around 900 MHz. This stray radiation can be picked up and/or conducted in lead wires from the magnetron to the isolation transformer and lead wires from the isolation transformer to an external heater supply inverter. The isolation transformer, which is designed to hold off 20 kV, provides no significant barrier to currents induced by the stray radiation.
Because of the high levels of stray radiation, it is usually necessary fully to shield the magnetron and the isolation transformer in a metallic or other electrically conductive screened chamber. If a filter is fitted, its effectiveness may be limited by radiation picked up on its output. Such a filter may provide no attenuation to the stray radiation because the filter itself acts as an antenna and picks up the stray radiation on its output even although the filter may have significant attenuation over the desired frequency band.
In many applications the drive current from the heater supply inverter is modulated as a high frequency (Fi) square wave, as illustrated in a second insert 21 in FIG. 1. Any filter used must be able to pass, without any significant distortion and loss, the heater supply inverter waveform into the screened chamber but significantly attenuate and minimise stray radiation to the outside of the screened chamber.
It is an objective of the current invention at least to ameliorate some of these difficulties in the prior art.